5G communication systems use massive multiple-input multiple-output (MIMO) technology and beamforming to provide higher data rates. Massive MIMO technology uses a very high number of antennas.
FIG. 1 illustrates an example of a massive MIMO system which uses beamforming. In FIG. 1, a next Generation Node B (gNB) transmits signals to several user equipment (UE). The gNB comprises a very high number of antennas. The antennas of the gNB are grouped into several antenna arrays. The gNB uses beamforming to transmit signals to the UEs. Similarly, the gNB receives signals from the UEs through the beams. The UEs access a network through the gNB. Since the gNB comprises a very high number of antennas, the gNB may provide higher data rates and have a higher beamforming gain.
FIG. 2 illustrates an example of a transmitter of a massive MIMO system which uses all-digital beamforming. In FIG. 2, the transmitter inputs Ns baseband signals to a baseband precoding block. The baseband precoding block outputs Lt precoded signals. The precoded signals are the inputs of Lt digital-to-analog converters (DAC). The Lt outputs of the DACs are the inputs of Lt radio frequency (RF) chains. The Lt outputs of the RF chains are transmitted through the Nt antennas of the transmitter. Similar to FIG. 1, the transmitter of FIG. 2 also uses beamforming. The baseband precoding block performs precoding in order to transmit the precoded signals in different beams.
All-digital beamforming has the following disadvantages: signal processing has high computational complexity; and high power consumption.
FIG. 3 illustrates an example of a transmitter of a massive MIMO system which uses hybrid digital/analog beamforming. Hybrid beamforming performs precoding in the digital domain and in the analog domain. Similar to FIG. 2, the transmitter inputs Ns baseband signals to a baseband precoding block. The baseband precoding block performs precoding in digital domain. The baseband precoding block outputs Lt precoded signals. The precoded signals are the inputs of Lt DACs. The Lt outputs of the DACs are the inputs of Lt RF chains. However, different from FIG. 2, in FIG. 3, the Lt outputs of the RF chains are the inputs of a RF precoding block. The RF precoding block performs precoding in the analog domain. The RF precoding block outputs Nt precoded signals, which are transmitted through the Nt antennas of the transmitter.
Comparing FIGS. 2-3, the number of baseband signals Ns is less than or equal to the number of RF chains Lt: Ns≤Lt. In FIG. 2, the number of RF chains Lt equals the number of antennas Nt: Lt=Nt. However, in FIG. 3, since hybrid beamforming has RF precoding, the number of RF chains Lt may be less than the number of antennas Nt: Lt<Nt.
In hybrid beamforming, the number of RF chains may be less than the number of antennas. Thus, hybrid beamforming may require fewer RF chains, while maintaining high beamforming gain and diversity order. Since hybrid beamforming may require fewer RF chains, hybrid beamforming may reduce the production cost of massive MIMO systems.
FIG. 4 illustrates an example of a transmitter which uses digital pre-distortion. In FIG. 4, the transmitter includes power amplifier (PA), a digital pre-distortion (DPD) module and a DPD adapter. The transmitter receives input x(n). The transmitter outputs y(n). Since the power amplifier causes distortion of output y(n), the transmitter uses DPD to cancel the distortion. The DPD module performs DPD on x(n) and outputs a pre-distorted signal to the PA and the DPD adapter. PA amplifies the pre-distorted signal and outputs y(n). The DPD adapter is coupled to the output of the PA to receive y(n) as a feedback signal. Since the DPD adapter receives the input and the output of the PA, the DPD adapter may estimate the distortion caused by the PA on y(n). After estimating the distortion, the DPD adapter may adjust the DPD module in order to cancel the distortion.
Power amplifiers are indispensable components of a communication system. Power amplifiers affect the overall performance and throughput of the communication system. However, power amplifiers are inherently non-linear. Thus, power amplifiers also cause the following problems: spectral re-growth; adjacent channel interference and out-of-band emissions; and in-band distortion. Therefore, communication systems require DPD to correct these problems.
The DPD adapter may use a model for the power amplifier (PA). The model may be the one shown in the following equation:
      y    ⁡          (      n      )        =            ∑              k        =        1              ⁢                  ∑                  q          =          0                    ⁢                        c          kq                ⁢                  x          ⁡                      (                          n              -              q                        )                          ⁢                                                        x              ⁡                              (                                  n                  -                  q                                )                                                                      k            -            1                              
Coefficients “c” represent the response of the PA. During PA model training, the DPD adapter may input known signals x(n) into the PA. The DPD adapter may receive the output y(n) of the PA. The DPD adapter may estimate the coefficients “c” with the following equation: c=(x*x)−1x*y. x* is the complex conjugate of the input signal x. (x*x) is the autocorrelation of the input signal x.
FIG. 5 illustrates an example of distortion suppression by digital pre-distortion. The transmitter may receive a signal. At the left of FIG. 5, FIG. 5 shows the original transmit signal spectrum. In the example of FIG. 5, the original transmit signal spectrum is a square spectrum occupying a frequency band. First, the transmitter may perform DPD at the DPD module. Then, the predistorted signal is input into the PA. The output of the PA is shown as a solid line. However, the impairments of the PA distort the spectrum, both inside the original frequency and outside the original frequency band. FIG. 5 also shows an example of the transmit signal spectrum if the transmitter does not perform DPD, which is shown as a dashed line. In FIG. 5, if the transmitter does not perform DPD, the PA would distort the spectrum of the original frequency band. In other words, the PA would cause in-band distortion. Moreover, the PA would distort and spread the signal outside of the original band. In other words, the PA would cause spectral re-growth and out-of-band emissions. Thus, in FIG. 5, DPD suppresses spurious spectrum, represented by the arrows from the dashed line to the solid line. DPD reduces spectral re-growth, out-of-band emissions, and in-band distortion.
Due to the benefits described above, employing DPD in a massive MIMO system is desired. However, DPD in a massive MIMO system also presents particular challenges. In a DPD method for a system with a single antenna, each amplifier requires a digital chain. Each digital chain comprises a DPD module and a feedback circuit. In a system with many antenna arrays, the number of power amplifiers is much higher, and DPD would require a very high number of digital chains.
Furthermore, in systems with antenna arrays, each digital chain is coupled to an antenna array. Several power amplifiers output signals to the antenna array. Since the number of power amplifiers is very high, it is desirable that DPD is performed for a feedback signal which combines the outputs of several power amplifiers. In DPD for conventional systems, a combined feedback signal is not needed since the number of antennas is not high.
The disclosure is directed to a transmitter with many antenna arrays using hybrid beamforming and DPD. The transmitter of the disclosure may comprise one single feedback circuit that may combine the feedback signals from the antenna arrays into one combined feedback signal. The feedback circuit is able to recover the signals output by the antenna arrays because the feedback circuit performs a code division method. Thus, the transmitter of the disclosure may perform DPD while reducing feedback circuit hardware complexity and cost.